Wireless system comprises various network systems including a Wireless Local Area Network (WLAN) system, a cellular network system, etc. In this document, for the convenience of explanation, the present invention is described as embodied in a WLAN system. However, it should be noted the WLAN system is used only for the exemplary purpose.
Standard for WLAN technology has been developed as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard. IEEE 802.11a and 802.11b use an unlicensed band at 2.5 GHz or 5 GHz. IEEE 802.11 b provides a transmission rate of 11 Mbps and IEEE 802.11 a provides a transmission rate of 54 Mbps. IEEE 802.11g applies Orthogonal Frequency-Division Multiplexing (OFDM) at 2.4 GHz to provide a transmission rate of 54 Mbps. IEEE802.11n applies Multiple Input Multiple Output (MIMO)-OFDM to provide a transmission rate of 300 Mbps for four spatial streams. IEEE 802.11n supports a channel bandwidth up to 40 MHz to provide a transmission rate of 600 Mbps.
Currently, IEEE 802.11af standard that specifies operation of an unlicensed device in a TV White Space (TVWS) band is being developed.
A TVWS includes an Ultra High Frequency (UHF) band and a Very High Frequency (VHF) band as a frequency allocated to a broadcast TV and refers to a frequency band in which use of an unlicensed device is permitted under the condition that it does not hinder communication of a licensed device operating at a corresponding frequency band. The licensed device includes a TV, a wireless microphone, etc. The licensed device may be called an incumbent user or a primary user.
Operation of all unlicensed devices is permitted in frequency bands of 512 to 608 MHz and 614 to 698 MHz except in a few special cases. However, in frequency bands of 54 to 60 MHz, 76 to 88 MHz, 174 to 216 MHz, and 470 to 512 MHz, only communication between fixed devices is permitted. The fixed devices refer to devices which transmit signals only in a determined location. An IEEE 802.11 TVWS terminal refers to an unlicensed device operating using an IEEE 802.11 Media Access Control (MAC) layer and a physical layer (PHY) in a TVWS spectrum.
An unlicensed device which desires to use a TVWS should provide a protection function for a licensed device. Accordingly, the unlicensed device must confirm whether the licensed device occupies a corresponding band before starting signal transmission in the TVWS.
The unlicensed device may confirm whether a corresponding band is being used by the licensed device by performing spectrum sensing. A spectrum sensing mechanism includes energy detection and feature detection. If strength of a signal received through a specific channel is above a prescribed value or if a DTV preamble is detected, the unlicensed may determine that the licensed device is using the specific channel. If it is determined that the licensed device is using a channel immediately adjacent to a currently used channel, the unlicensed device should lower transmission power thereof.
However, if a station (STA) operating as an unlicensed device depends only on frequency sensing to acquire available channel information, the STA may encounter an increased burden due to a sensing action for a TVWS and a procedure may be delayed. Meanwhile, an unlicensed device may obtain available channel information information in a corresponding area by accessing a geolocation Database (DB) through the Internet or a dedicated network. The geolocation DB stores and manages information about registered licensed devices and information about available channels which dynamically vary according to geographic location and channel use time of the licensed devices.
As described above, available channels within a TVWS dynamically vary over time. However, since an STA operating as an unlicensed device in a TVWS should use only available channels to protect a licensed device, it should determine whether available channels have been changed.